1. Field of the Invention
The invention is concerned with a temperature-responsive record material for use in thermographic recording and reproducing systems and, more particularly an improved heat-sensitive record material comprising a supporting sheet provided with a heat-sensitive composition containing, in a single layer, mark forming components which react to produce a mark according to a selectively applied temperature pattern.
More particularly, the record element referring to the invention is adapted to be used for the data record in the systems wherein there are used as printing means, heated metal pins, thermic heads or any other device that transmits thermic energy.
2. Description of the Prior Art
Known in the literature there is the existence, which is therefore no object of the invention, of some colorless basic chromogenic compounds capable of being transformed into the colored form when coming into contact with acid or ionizing environments.
These compounds are, usually, leuco-compounds of triphenylmethane and fluorane dyestuffs having a lactonic structure and, among the most known thereof, there are cited here below: - 3,3-bis-(4-dimethylaminophenyl)-6-dimethylphthalide (Crystal Violet Lactone or CVL), giving a colour from blue to violet; - 3,3-bis-(p-dimethylaminophenyl)-phthalide or (Malachite Green Lactone or MGL), giving a green color; - Xanthene-9,0-benzoic acid, 3,6-bis-dimethylamino-9-p-nitroaniline lactam, giving a red color; - N-(p-nitrophenyl)-Rhodamine B-lactam (RBL) and 3',6'-bis-diethylaminofluorane both giving a red color.
In order to establish the acid or ionizing environment needed for their reaction, there are usually employed tannic acid, gallic acid, phenols, polyphenols and phenolic resins, anhydrides, anilides, imides, attapulgite, silica, etc.
It is furthermore known the use, for thermographic purposes, of layers containing, as a thermosensitive compound, mixtures of a basic chromogenic compound and an acidic compound. By selectively applying heat to said layers, the acidic compound melts thereby giving the suitable medium for turning the basic chromogenic compound into its colored form.
Nevertheless, temperature-responsive elements as above described show a marked tendency to develop color also in the absence of heat, so that this phenomenon already occurs by operating the mixture of the single dispersions comprising respectively the basic chromogenic compound and the acid compound.
In fact, the dispersions of CVL and acidic compounds, obtained by using water-soluble binders like casein, starch, modified starchs, pectine, polyvinyl acetate/crotonic acid copolymers, alkali-soluble phenolformaldehyde resins, polyvinylpyrocolidone and copolymers of it, gum arabic, urea-formaldehyde resins, etc. when mixed, give immediately a greenish color turning rapidly to a hell blue and dark blue color. In many cases, already the CVL dispersion appears greenish colored.
Besides the above cited inconveniences the specified thermographic layers also show a poor resistance to abrasion and to the operative conditions, owing to the presence of the water-soluble binders and to the low binding power of the colloid used in the dispersion.